This invention relates generally to processes for extracting hexavalent chromium from aqueous solutions while retaining its oxidation state, and more particularly to processes which allow easy recovery and optional reuse of hexavalent chromium from extracted aqueous solutions.
Many industrial processes generate waste containing chromium in the hexavalent charge state. Examples include, but are not limited to, aqueous waste streams, sludges, and solids from metal fabrication operations including chrome plating, anodizing, conversion coating, and phosphating, from paints and paint extracts, from the manufacture of polymers and plastics, from leather tanning, from the manufacture of wood preservatives, fungicides, and other pesticides, and leachates from landfills and contaminated sites.
Waste from electroplating operations is fairly well characterized. The chromium metal finishing and plating process lines in many commercial plating shops are arranged so that the plating rinse tanks containing hexavalent chromium are countercurrent rinses. The concentration of Cr(VI) in these waste rinse waters is very low. The greatest amount of hexavalent chromium in the waste of most plating shops is found in the process dump tanks. Separate extraction of the hexavalent chromium in dump tanks combined with extraction of the hexavalent chromium in plating rinse tanks could reduce the overall release of chrome from a given operating facility by more than 95%.
The waste water from chromium plating operations contains other materials in addition to hexavalent chromium. High concentrations of sulfates are present because high concentrations of sulfuric acid are used in the plating baths. There are also minor concentrations of other chemicals, including ions of trivalent chromium, chloride and fluoride ions, ferric and ferrous ions from the steel base metal, lead from electrodes, and barium ions. There may also be minor amounts of mixed hydrocarbons from the steel base metal.
The pH of the waste waters is typically between 0.5 and 2.5, and the temperature is generally between about 40xc2x0 and 60xc2x0 C.
Other aqueous hexavalent chromium waste streams exhibit similar characteristics. Anodizing or phosphating rinses used in metal treating can be composed solely of hexavalent chromium in solution at a temperature of about 50xc2x0 C. Metal surface conversion coating wastes may also contain other ionic and dissolved components inherent in the original solution, e.g., fluorides, molybdates, or ferricyanides.
One method currently used to treat waste streams containing hexavalent chromium is to reduce the hexavalent chromium to the trivalent oxidation state with concurrent elevation of the pH. This process results in the reduction of Cr(VI) to Cr(III) and the formation of copious amounts of mixed Cr(III) oxide/hydroxide precipitates. Ferrous salts or tetravalent sulfur species are used most frequently to reduce Cr(VI) to Cr(III). Ferrous salts may be added alone or in combination with a water-soluble sulfide, such as sodium sulfide, to the solution. Metallic iron reportedly performs the same reduction of the Cr(VI) ion, although at a much slower reaction rate. When ferrous species are used as the reducing agent, the precipitate formed is even more voluminous due to the formation of a mixed Cr(III)-Fe(III) hydroxide. The use of sulfites (SO32xe2x88x92) or metasulfites (S2O52xe2x88x92) helps to alleviate this problem somewhat, but the fluffy precipitated flocs are still difficult to aggregate into a dense compact precipitate.
Reduction of hexavalent chromium has been successfully implemented using electrolysis, although the energy costs increase significantly as the concentration of chromium decreases. Reduction of hexavalent chromium has also been demonstrated using formaldehyde, xanthates, and anaerobic microorganisms in solution.
As a practical matter, processes which result in the reduction of the hexavalent chromium to the trivalent oxidation state preclude reuse of the chromium. For example, the Cr(III) hydroxide made using conventional reduction/precipitation treatment approaches is in a form from which recovery is not cost effective. If an iron reduction process is used, the Cr(III) formed is intimately mixed with Fe(III). Even if the Cr(III) hydroxide is prepared in a xe2x80x9cpurexe2x80x9d form, such as from bisulfite reduction, the voluminous hydroxide precipitate would then need to be resolubilized and reoxidized to hexavalent chromium. This would not be practical in a plating facility because of the enormous energy input required for reoxidation.
Technologies have been developed for removal of hexavalent chromium in a form that conceptually allows for recovery and reuse. For example, anion exchange has been used to remove hexavalent chromium, but the process is difficult to control under the harsh conditions found in plating waste streams. Similar operational difficulties have been encountered with carbon absorption processes. Evaporation has also been used, but it requires a tremendous energy input or a long time, which renders the technology impractical for plating facilities.
In light of the difficulties associated with existing treatment and recovery processes, different chromium electroplating precursors have been examined, including soluble trivalent chromium. However, these alternatives have not been generally accepted yet, and hexavalent chromium is currently the preferred plating precursor.
Hexavalent chromium is extremely toxic. As a result, the United States Environmental Protection Agency (U.S. EPA) limits discharge of hexavalent chromium to 5 ppm, and the limits in some state and local jurisdictions are even lower. In addition, the current cost of disposing of a 55 gallon drum of waste containing hexavalent chromium is on the order of several hundred dollars.
Therefore, there is a need for a method of recovering and optionally reusing the hexavalent chromium in waste streams.
The present invention meets this need by providing a process for recovery of hexavalent chromium from streams. The method includes providing a stream containing hexavalent chromium, reacting a soluble non-toxic precipitating reagent with the hexavalent chromium to form an insoluble precipitating reagent-chromate precipitate, and recovering the insoluble precipitating reagent-chromate precipitate. The process optionally includes reacting the insoluble precipitating reagent-chromate precipitate with an acidic solution to form an insoluble precipitating reagent precipitate and a soluble hexavalent chromium compound, and recovering the soluble hexavalent chromium compound. The process may also include reacting the insoluble precipitating reagent precipitate with a solubilizing reagent to form the soluble non-toxic precipitating reagent.
The soluble non-toxic precipitating reagent includes, but is not limited to, bismuth compounds; lanthanide compounds; aluminum compounds; titanium compounds; trivalent chromium compounds; organic compounds containing a group selected from nitronium, phosphonium, sulfonium, stibonium, iodonium, pyrylium, or combinations thereof; or combinations thereof.
The acidic solution includes, but is not limited to, sulfuric acid, hydrochloric acid, hydrofluoric acid, boric acid, phosphoric acid, pyrophosphoric acid, phosphomolybdic acid, phosphotungstic acid, silicomolybdic acid, silicotungstic acid, or combinations thereof. The acidic solution includes, but is not limited to, acidic oxalates, malonates, succinates, maleates, fumarates, malates, tartrates, salicylates, or combinations thereof.
The insoluble precipitating reagent-chromate precipitate may have a solubility in water of less than 1xc3x9710xe2x88x922 M Cr+6 (500 ppm Cr+6). Optimally, the insoluble precipitating reagent-chromate precipitate may have a solubility in water of less than 5xc3x9710xe2x88x923 M Cr+6 (250 ppm Cr+6)
The solubilizing reagent includes, but is not limited to, nitric acid, perchloric acid, sulfuric acid, hydrochloric acid, acetic acid, propionic acid, lactic acid, citric acid, or combinations thereof